The problem of creating at least three degree-of-freedom (3D) tracking devices is a long-standing one. There has been a variety of attempts to determine the position and movement of a target. For example, global positioning system (GPS), receives a signal from at least four satellites and provides the position of the receiver. Loran C operates on a similar principle, but is based on ground deployed radio beacons. Many tracking systems have developed to track moving vehicles. Most of these systems employ either directional antennas or rely on a comparison of the phase of the arriving signal to the different parts of a multi-section antenna. While these systems perform well in their applications, either speed, accuracy or cost may prohibit their employment in real time computer applications.
A relatively large number of electromagnetic (EM) trackers are available for computer applications. Most of these EM trackers are based on near field EM propagation. Polhemus Incorporated pioneered the field of AC magnetic trackers and holds many patents since 1977. Ascension Corporation has developed a DC magnetic tracker that is less prone to interference from metal. A variety of ultrasonic trackers is also known. In the field of optical tracking, the pioneering work done at the University of North Carolina has shown the efficacy of this method. Mechanical devices and a combination of inertial-global positioning systems have also been developed to determine the position and orientation of objects.
While research is still being conducted in all of these fields, these technologies are relatively mature. However all of these techniques, while highly evolved, are subject to limitations that are inherent to each method. No single current technology is able to meet the requirements demanding computer applications require.
Prior systems also are subject to the limitation that a receiver must be located within a line of sight of each position in which the transmitter is to be tracked. That is, in buildings, a receiver is required in each room and any connecting halls that are not within a single line of sight. This restriction requires that a significant number of receivers be employed. The high number of receivers increases the cost of the system as well as installation and maintenance costs.
The need for a multidimensional tracking sensor is rapidly growing, and expanding into previously unrelated technologies. Specifically, the recent development of virtual reality equipment has generated an emphasis on a short range tracking sensors for a helmet mounted display (HMD). In addition, these short range tracking sensors are finding applications in automobile crash testing where geometrical data is logged directly during the test. Further, the short range tracking sensors may find application in the medical field for rehabilitation and injury claims where the device can track movement of the human body. The variety of uses for multidimensional tracking sensors is very diverse, and includes animation, tele-operation, and training simulation. As the enabling hardware technology becomes further refined, it is anticipated the number of applications will multiply. The recent increases in performance and cost effectiveness of digital signal processing and data converter products have increased the feasibility of electromagnetic (EM) correlation techniques in the field of multidimensional position tracking.